Natural and calcined clays are used as opacifying pigments and extenders of TiO.sub.2 (titanium dioxide) in paper filler applications, paper coating and paint formulations. The opacifying strength of a pigment is expressed by a measurable property, the so-called "Pigment Scattering Coefficient," Sp, which is based on the Kubelka-Munk theory. The pigment scattering coefficients are determined by measuring the absolute reflectances, R.sub.0 (black body backing) and R.sub.w (white body backing of known reflectance), of thin, uniformly deposited layer of pigment on a substrate of known reflectances. Clay pigments are much less expensive than prime pigments, such as TiO.sub.2 ; and their pigment scattering coefficients, that is, opacifying strength, are considerably lower than those of TiO.sub.2 (see Table I, Pigment Scattering Coefficients, Sp):
TABLE I ______________________________________ PIGMENT SCATTERING COEFFICIENTS (Sp) cm.sup.2 /g. ______________________________________ Clays 670-1,250 Calcined Clays 1,800-2,150 TiO.sub.2 (Anatase) 4,000-4,800 TiO.sub.2 (Rutile) 4,600-5,100 U/F Pigments 3,500-4,200 ______________________________________
Peter Economou, John F. Hardy and Alfred Renner, in a recent U.S. Pat. No. 3,909,348, described the high opacifying properties of urea-formaldehyde pigments in paper filler applications. The pigment scattering coefficients, Sp, of urea-formaldehyde pigments approach those of TiO.sub.2, anatase grade. However, the urea-formaldehyde pigments are equally as expensive as TiO.sub.2.
Dominic Simone, of NL Industries, Inc., describes in his U.S. Pat. No. 3,912,532 the opacifying properties of clay coated with urea-formaldehyde particles in paper filler and paper coating applications. Simone in his invention describes clay particles which are encapsulated with a coating of finely divided particles of urea-formaldehyde polymer, which have been formed and polymerized onto the surface of the clay particles. Simone pretreats well dispersed clay particles in water with urea-formaldehyde prepolymer and then adds the acid catalyst to polymerize the ure-formaldehyde prepolymer.
The present invention relates to the high opacifying strength of flocculated clay particles (as opposed to dispersed clay mentioned in the Simone patent) which are treated with an acid to form "structured agglomerates," which are then stabilized with urea-formaldehyde polymeric particles. The acid used to flocculate the clay particles is also the catalyst for the urea-formaldehyde polymerization. Due to the very thixotropic character of the flocculated clay particles in water and, in an attempt to produce products with "open structure" or maximum possible number of microvoids of desirable size, th solids of clay slurries and of the urea-formaldehyde prepolymer of this invention are considerably lower than the concentrations stated in Simone's examples. As a result of these major differences between Simone's invention and this invention, the pigment scattering coefficients of products made by this invention are considerably higher than the pigment scattering coefficients of the products made by Simone's inventioon, when both products have the same clay/urea-formaldehyde polymer weight ratio.